This invention relates to a dipole ring magnet for use in either a magnetron sputtering apparatus or a magnetron etching apparatus to produce a magnetic field in the direction of a diameter of the ring magnet. Essentially the dipole ring magnet is an assembly of a plurality of anisotropic permanent magnet segments in a circumferential arrangement.
In the recent electric and electronic industries, magnetron sputtering and magnetron etching are widely employed since the efficiency of sputtering or etching can be greatly enhanced by utilizing a high-density plasma generated by a discharge of electricity in a magnetic field.
A permanent magnet unit is essential to a magnetron sputtering or etching apparatus. For example, in conventional magnetron sputtering apparatus of a planar diode type it is usual to use a magnet unit consisting of an annular permanent magnet, another permanent magnet in the shape of a disk which is coaxially placed inside the annular magnet and a yoke plate which connects one end face of the disk magnet to one end face of the annular magnet. The magnet unit is adjacent to one of the planar electrodes, and a planar target is placed on this electrode to face a substrate on the opposite electrode. Accordingly a doughnut-like magnet field is produced within the space between the two electrodes, and a high-density plasma is generated over the target surface by the interaction of the magnetic field and an electric field in the direction parallel to the axis of the annular magnet.
However, the above described permanent magnet unit has a disadvantage in respect of the uniformity of the magnetic field. That is, in a horizontal plane close to and parallel to the target surface, a component of the magnetic field that is parallel to the target surface and normal to the direction of the electric field does not make a uniform distribution. Such a component of the magnetic field will be referred to as horizontal magnetic field. In the plane in question the strength of the horizontal magnetic field greatly varies with radial distance from the center of the target on the center axis of the annular magnet, and accordingly the density of the plasma over the target surface becomes very uneven. Since the sputtering efficiency depends on the density of the plasma the target is unevenly consumed, and consequently the utilization efficiency of the target lowers. This is a serious problem from an economical point of view since in most cases the target is an expensive material.
In the case of a magnetron etching apparatus using the above described permanent magnet unit, the unevenness of the plasma density causes uneven etching such as locally excessive etching of a wafer. Besides, there is a possibility of breakage of the wafer by a distribution of potential within the wafer surface induced by the uneven density of the plasma.
In view of the above problems there is the idea of using a dipole ring magnet to produce a uniform magnetic field for magnetron sputtering or etching. The dipole ring magnet has a plurality of pillar-like magnet segments each of which is an anisotropic permanent magnet magnetized in a direction normal to its longitudinal axis. The pillar-like magnets are held in a circumferential arrangement by a nonmagnetic retainer in the form of a hollow cylinder, and the pillar-like magnets are differently oriented so as to produce a magnetic field in the direction of a diameter of the ring magnet within the cylindrical space in the ring. In a plane close to the middle cross section of the dipole ring magnet the magnetic field becomes a horizontal magnetic field, and the uniformity of this horizontal magnetic field is very good by comparison with the horizontal magnetic field produced by the above-described conventional permanent magnet unit.
However, a problem remains in practically using the dipole ring magnet for magnetron sputtering or etching. That is, when the length of the dipole ring magnet (length of the pillar-like anisotropic magnet segments) is relatively short the horizontal magnetic field inside the ring magnet is not sufficiently uniform. The strength of the horizontal magnet field becomes lowest around the center axis of the ring magnet and gradually rises as radial distance from the center axis increases. Furthermore, the rate of such a variation in the field strength varies with the angle of radius with the direction of the magnetic field. It is possible to sufficiently improve the uniformity of the magnetic field by lengthening the dipole ring magnet, but for application to practical magnetron sputtering or etching apparatus it is difficult to desirably lengthen the dipole ring magnet.